Field of the Invention
The present invention relates to an electron gun employing an indirectly heated cathode, gate electrode and anode.
Electron guns constructed for and used for processing workpieces have been generally known and there exist many publications relating to such electron guns.
A majority of conventional electron guns are based on directly heated cathodes. These cathodes are heated separately by a heating voltage, and an high voltage for electron escape from the cathode surface and for electron acceleration is applied to cathode independent of a heating means for the cathode. This separation of heating voltage and electron acceleration voltage is necessary in connection with the required overlay of the two electrical voltage variables. The electrons emitted from the cathode can only be controlled with some effort due to the high voltage involved. Furthermore, the only cathode type that can be used with direct heating is a band cathode. Such constructions have been described, for example, in U.S. Pat. No. 386,222, U.S. Pat. No. 3,433,922, and U.S. Pat. No. 4,317,983.
A combined laser and plasma arc welding torch is taught in U.S. Pat. No. 5,700,989 and 5,700,785 to Dykhno et al. In each case the laser beam passes through a hole in a cathode. U.S. Pat. No. 3,621,324 to Fink teaches a high power cathode. The bolt electrode is heated by electron bombardment.
Indirectly heated cathodes are associated with the advantage of in general allowing a longer service life also allowing more independence for geometrical shaping of the electron gun configurations. Known constructions employ electric heating systems and in particular resistance heating as an energy source for indirect heating. Such configurations are presented in the printed patent documents European Patent Document EP 0,416,535, European Patent Document EP 0,505,211 and German patent Document DE 44,43,830.
High electric currents in the electric heater are required to achieve a sufficient temperature that will cause emission of electrons from the cathode. Moreover, the resistive sources of heat are non-focussed such that a major part of the thermal energy generated by the heater does not reach an appropriate cathode surface for electron emission and a large part of the heat generated in the heater is dissipated into the space of the electron gun and to the casing of the electron gun. The uncontrolled dissipation of thermal energy by the electric heater results in an increase of the electric energy required to achieve the required cathode temperature. This is why indirectly heated electron guns can only be used efficiently in lower-power electron guns. Indirectly heated electron guns are less suited for high-power electron guns. There is only limited control of the generation process of electron emission due to the non-directional thermal radiation in case of an indirect source of heat.
Band cathodes are conventionally heated directly by means of an electric resistor heating element with electrical consumptions of for example 8 volts and 10 amps. Such electric resistors encountered difficulties with the connections to the power source, because the connection resistance would frequently be higher than the resistance of the heating element.
Purposes of the Invention
It is an object of the present invention to furnish an electron gun which is convenient to control in its output characteristics.
It is another object of the present invention to eliminate problems in electron guns associated with high heating currents for the cathode.
It is yet another purpose of the present invention to eliminate problems in electron guns associated with heating of the junctions to the cathode heater.
It is a still further purpose of the present invention to avoid a magnetic field generated by a heating current for the cathode heater from interfering with the direction and/or bundling of the emitted electron beam.
These and other objects and advantages of the present invention will become evident from the description which follows.
The present invention provides an electron gun comprising an indirectly heated cathode, a gate electrode and an anode, for generating electron beams of various shapes and power that are preferably used for processing workpieces.
The electron gun comprises a housing, a cathode having an emission surface on a first side and an irradiation surface on a second side of the cathode disposed opposite to the first side and said cathode being mounted in the housing, a gate electrode disposed adjacent to the cathode for controlling the beam of electrons emitted by the cathode and mounted in the housing, an anode mounted in the housing and disposed at an appropriate distance from the cathode for building up a voltage between cathode and anode and for accelerating electrons emitted by the cathode, a source of a laser beam for directing a laser beam to the irradiation surface of the cathode.
The source of the laser beam is a member selected from the group consisting of solid-state laser, an optical facility to decouple laser beams and combinations thereof. The source of the laser beam is placed opposite to a surface other than the emission surface of the cathode. The side disposed opposite to the emission surface of the cathode is located in the laser beam path.
A member selected from the group consisting of a photodetector, a solid-state image sensor, an optical fiber waveguide connected to a photo detector, an optical fiber waveguide connected to a solid-state image sensor and combinations thereof is placed opposite to a surface of the anode and in the path of the light of the laser beam. The source of the laser beam and the member selected from the group consisting of a photodetector, a solid-state image sensor, an optical fiber waveguide connected to a photo detector, an optical fiber waveguide connected to a solid-state image sensor and combinations thereof and the member selected from the group consisting of a control unit, a closed-loop control system and combinations thereof are interconnected.
The source of the laser beam can be an optical facility to decouple a laser beam together with and optically connected to a laser beam generating facility through at least one optical fiber. The laser beam generating facility for generating a laser beam is disposed outside of the housing.
An optical fiber can be located in the waveguide. A hole of the cathode anchor forms a direct electroconductive connection to the waveguide.
A high voltage plug is connected to the power supply unit. The waveguide is a component of the high-voltage plug. A first end of said waveguide is placed outside the housing wherein and a second end of said waveguide is disposed at a distance from about 2 to 150 millimeters relative to the cathode or inside the cathode anchor. The waveguide can be furnished with an electroconductive connection to the power supply unit for the cathode.
The facility generating the laser beam can be a solid-state laser.
The facility generating the laser beam can be connected to the optical fiber through a member selected from the group consisting of a spherical lens, a half sphere, a taper, a two-sided beveling of the optical fiber, and combinations thereof.
The optical facility to decouple laser beams can be connected through the fiber laser to a light beam generating facility that functions as a pumping source. The cathode can be located in the laser beam path.
The optical facility to decouple laser beams can be a member selected from the group consisting of the end of at least one optical fiber, a half sphere placed at the end of said optical fiber, a lens placed in the downstream beam path from the end of said optical fiber, and combinations thereof. The half sphere can be melted on and comprises a cast resin.
The facility to decouple laser beams can comprise two lenses placed at a distance of from about 5 to 50 millimeters relative to one another. The cathode can be a member selected from the group consisting of a band cathode, a band cathode with an electrode emitting body attached to it, a pin cathode, and combinations thereof. A surface of said pin cathode can comprise an indentation and/or a prominence and/or a projection and/or a protrusion, and wherein the electron-emitting surface is circular. The electron-emitting component of the pin cathode can be a bolt.
Measuring equipment can be used to determine laser power and the resulting cathode temperature. A member selected from the group consisting of photodetector, solid-state image sensor, a control unit, a closed-loop control system, and combinations thereof can be part of the measuring equipment.
An electron gun according to the present invention is furnished with a cathode heated indirectly by irradiation with a laser beam, with a gate electrode, and with an anode. No heating current flows through a cathode constructed in this way. Thus a cathode of the most varied geometrical shape can be used. Massive cathodes such as bolt-type cathodes can be employed in addition to band cathodes and band cathodes with bodies attached to them.
Using a massive body for a cathode results in a longer service life of the cathode as compared to a band cathode. The surface opposite the emission surface-should be the target of the irradiating laser beam. Thus the indirect cathode heating by irradiation is implemented in a simple construction.
The electron gun construction according to the present invention is associated with the further benefit of rendering the service life of the irradiative source of heat for the cathode is identical with the service life of the laser used. It is particularly advantageous to place the laser outside the housing, as this ensures a very long service life of this source of heat.
An indirect temperature measurement of the cathode allows control of the emission of electrons and thus an improvement of the overall emission property of the cathode. The effect of craters that may develop on the emission surface of the cathode can be compensated by for example adjusting the irradiation with the laser beam.
According to the present invention, it is further disclosed to place a facility for generating the laser beam outside the housing of the electron gun. In this way, the most varied lasers may be used. The type of the laser can be selected in accordance with the cathode temperature required for machining workpieces. This permits an efficient use of both the source of heat and the cathode. It can be easy to configure the electron gun depending on the type of workpieces to be machined and the number of workpieces to be processed. If the workpieces always remain the same, an electron gun can be constructed specifically adapted to these workpieces and constructed highly specialized, but such electron gun can easily be adapted to differing requirements when jobs change. This underlines the universal applicability of the electron gun according to the invention.
The geometric dimension of the cathode does not depend on the size of the housing of the electron gun only the optical facility to decouple laser beams and a section of the optical fiber are inside the housing thereby enabling to employ a very small size of an electron gun down to a miniature construction.
It is particularly advantageous in this context to use a solid-state laser that has the shape of a diode laser suitable for development.
Supply of an electrical potential to the cathode via an electroconductive waveguide into which the optical fiber is integrated ensures that the potential of the cathode is applied to the optical fiber. The optical fiber has to be placed on substantially the same potential as is the electrical potential of the cathode. The result of this is that the electron beam runs only towards the anode.
An integration of the electroconductive waveguide into a high-voltage plug results in a unit that can be handled easily. At the same time, the integration provides for a compact voltage potential supply to, and heating of the cathode. The required airtight leads to the electron gun are considerably reduced. Technological and economic expenditures required for the electron gun according tot he present invention are minimized.
Further ways to improve the efficiency of input into the optical fiber of the facility generating the laser beam is a direct connection, a connection via a spherical lens, a half sphere, a taper, or a two-sided beveling of the optical fiber.
The approximate coupling attenuations are as follows:
(5 to 8) dB for direct connection,
1.5 dB via spherical lens,
(0.2 to 1) dB via a half sphere,
(0.2 to 1) dB via a taper, and
1 dB for a two-sided beveling.
Such construction allows for a selection dependent upon the facility generating the laser beam, the cathode temperature to be achieved, or the most sensible solution in technological and economic terms.
A favorable embodiment is obtained when using a fiber laser. The fiber laser is pumped via an external facility that generates light beams and is located outside the housing of the electron gun. The optical fiber acts as an amplifier of the light beam coupled into it from the pumping source. Preferably, a laser diode should be used as a pumping source.
Other favorable embodiments of the optical facility to decouple laser beams are the end of at least one optical fiber itself, a half sphere placed at the end of an optical fiber, or at least one lens placed in the beam path downstream from the end of said optical fiber.
A most simple way of coupling the laser beam to the cathode is via the end of the optical fiber itself. A spacing provided between the end of the optical fiber and the cathode surface should be minimal for keeping radiation intensity losses as low as possible. Consideration of the high negative potential of the cathode should not be neglected. This is why the optical fiber is connected to the cathode potential at least at the end pointing towards the cathode. This coupling design is a most favorable embodiment in a technological and economic respect. No additional brackets and components for facilities to decouple laser beams are required. At the same time, there are no losses due to reflection and absorption at the boundary layers of the additional components.
A half sphere or lens placed at the end of the optical fiber acts as collimator, i.e. divergent laser beams that emerge from the optical fiber are converted into approximately parallel laser beams. This creates a point source.
The half sphere is either directly connected to the end of the optical fiber, or comprises the end of the optical fiber. The half sphere and the optical fiber form a unit requiring no additional fixing devices for the facility to decouple the laser beam. At the same time, placement of this unit opposite one cathode surface is less complicated. Moreover, the spacing can easily be adapted to the geometry of the cathode surface by changing the geometrical position of the optical fiber.
An embodiment of the half sphere that can be produced easily is either the molten end of the optical fiber itself, or an integrally cast resin half sphere. Another benefit is that no additional components are required in connection with the fiber. Thus the optical fiber can easily be adjusted to the cathode surface.
If a second lens is placed into the laser beam path after the first lens, then the laser beams are focused, and an intensity maximum is created in the focus point. Focusing also allows matching of the laser beam diameter and the diameter of a bolt-shaped cathode. Thus the circular surface of the bolt-type cathode is heated up by irradiation with the impinging laser beam.
An advantageous embodiment of the cathode includes a band cathode, a band cathode with an electron emitting body attached to it, or a body. Use of the body, either attached to the band cathode or just by itself increases the service life of the cathode as compared to a regular band cathode.
A minimum of one indentation or one prominence on the surface of said body used as a cathode, combined with a fixing device that at least partially receives the body of the cathode and comprising either at least one indentation of equal construction or one prominence of equal construction is a simple way to place the cathode in a fixed position inside the housing of the electron gun. This ensures constant radiation conditions.
A bolt-shaped cathode has a circular emission surface. An approximate circular electron beam is thus generated in interaction with the gate electrode and anode which ensures constant, direction-independent machining conditions for workpieces.
Measuring equipment containing the photodetector and solid-state image sensor and/or the control unit or closed-loop control system allows optimum tracking and control of the machining process. It is particularly advantageous to use this electron gun construction in machine-tools that are used universally, i.e. that are used to machine workpieces of various geometric shapes and materials that change constantly or at regular intervals.